The present invention relates generally to the field of fluid filtration. More specifically, the present invention relates to cylindrical disposable filter cartridges and methods and apparatus for making the same, whereby the filter cartridge includes a core element and a non-woven, melt-blown filter medium surrounding the core element.
Filter cartridges formed of a nonwoven mass of melt blown, indefinite (continuous) length polymeric fibers are well known and have achieved widespread use in fluid filtration applications. Typically, such melt blown filter cartridges are made by extruding a polymer through orifices associated with a melt blowing die to form continuous length fibers which are directed toward an axially elongate rotating mandrel. During melt blowing, a flow of inert gas (e.g., air) acts on the molten fibers so as to attenuate the fibers to relatively fine diameter and to randomly distribute the attenuated fibers onto the mandrel. Over time, therefore, an annular mass of nonwoven, randomly intermingled solidified fibers builds up on the mandrel. Controlled axial movement of the built-up mass of melt blown fibers relative to the melt blowing die will therefore allow a cylindrical filter cartridge of indefinite length to be formed continuously.
Such melt-blown filter cartridges have been proposed in the past to be produced continuously with and without a separate core element. For example, coreless melt-blown filter cartridges are known in the art through U.S. Pat. No. 5,340,479 to Szczepanski et al (hereinafter xe2x80x9cthe Szczepanski et al ""479 patent). According to the Szczepanski et al ""479 patent, coreless melt-blown filter cartridges are formed by directing streams of polymeric fibers toward a rotating mandrel which serves to support the melt-blown polymeric filaments during the production process. The resulting tubular filter portion is subsequently axially withdrawn from the mandrel and will include a central support zone of non-woven polymer filaments and a filtration zone of non-woven polymeric filaments surrounding the central support zone.
Tubular filter cartridges having separate central core elements are also known. In this regard, the attenuated streams of polymeric fibers are typically melt-blown onto a rotating and axially translating core element where the fibers are allowed to build up into a non-woven fibrous mass. The core element thus forms an integral central support structure for the non-woven fibrous mass during both the production process and later during filtration when the filter cartridge is placed into service. The formation of such filter cartridges with integral core elements is, however, somewhat problematic on a continuous production basis.
Several proposals exist in the art for producing filter cartridges having integral separate core elements on a continuous production basis. For example, U.S. Pat. Nos. 4,112,159 and 4,116,738 to Pall (hereinafter xe2x80x9cthe Pall ""159 and ""738 patentsxe2x80x9d, the entire content of each being expressly incorporated hereinto by reference) disclose the temporary end-to-end joining of sequential preformed core elements by means of coaxially interdigitated spacers so that the joined preformed core elements are capable of being rotated and axially traversed as a unit relative to a melt-blowing die during the continuous production of melt-blown filter cartridges. The melt-blown fiber layer is subsequently cut at about the midpoint of the spacers leaving a lap which extends beyond the core elements at each end, thereby making it possible to pull off a filter length by withdrawing the core portion of the next following spacer. Presumably, the spacers removed from the filter lengths are then reused in the process disclosed in the Pall ""159 and ""738 patents.
As an alternative to using preformed core elements, the Pall ""159 and ""738 patents also disclose that the core element can be formed in situ by means of a continuous rotatable tubular extrusion die. The core element is thus extruded continuously in tubular form with an open central passage, in a continuous length. Prior to receiving the melt-blown fibers, the extruded core element is perforated or slit by cutting means to provide a plurality of apertures for passage of fluid therethrough into the central open passage of the core.
The techniques disclosed in the Pall ""159 and ""738 patents are not without disadvantages. For example, when using spacers as the means to couple preformed core elements in an end-to-end manner, care must be exercised that the melt-blown fiber media is cut at about the spacer""s midpoint, thereby limiting the maximum length of the filter cartridge to the length of the preformed core element. Furthermore, the cut cannot be made entirely through the melt-blown fiber media and the spacer, since to do otherwise would result in sacrifice of the spacer thereby adding to the overall production costs of the filter cartridge. On the other hand, the continuous extrusion of the core element necessarily involves the provision of a rotatable extrusion die and core-perforation equipment which may not be cost effective in terms of already preformed core elements.
More recently, a technique for producing continuously melt-blown filter cartridges having preformed core elements has been disclosed through U.S. Pat. No. 5,653,833 to Mozelack et al (hereinafter xe2x80x9cthe Mozelack et al ""833 patentxe2x80x9d, the entire content of which is expressly incorporated hereinto by reference). According to the Mozelack et al ""833 patent, preformed thermoplastic elements are friction-welded coaxially in an end-to-end manner so that the core elements are joined integrally one to another. The integrally coaxially joined preformed core element can thus be rotated and traversed as a unit relative to the melt-blowing die(s) during the continuous production of indefinite length cylindrical melt-blown filter cartridges. Following melt-blowing of the polymeric fibers onto the joined core elements, the filter cartridge preforms may be cut to desired lengths (preferably on-line).
Broadly, the present invention relates to filter cartridges having a core element formed of a non-filtering, self-supporting non-woven mass of indefinite length continuous synthetic polymeric fibers, and at least one annular filtration zone layer formed of a mass of non-woven indefinite length continuous synthetic polymeric fibers, and to the apparatus and methods for making such filter cartridges.
Most preferably, the non-woven core element is formed in situ during filter cartridge manufacturing by melt-blowing synthetic polymeric fiber streams toward a forming mandrel. The fibers forming the core element are of sufficient diameter and/or density to define a highly open cross-sectional non-woven matrix structure which does not serve a filtration function under normal filtration conditions, but yet impart sufficient axial and radial rigidity to the core element sufficient to support the filtration zone fibers during manufacture and during filtration. That is, once formed, the core element provides a formation mandrel onto which the subsequently melt-blown filtration zone fibers may be accumulated.
Importantly, the non-woven fibrous mass of the core element must be completely solidified prior to the filtration zone fibers being melt-blown thereon. That is, according to the present invention, the filtration zone fibers are predominantly mechanically interlocked with the fibrous mass forming the core element, instead of being melt-bonded thereto. In such a manner, the relatively open pore structure of the core element can be maintained in the finished filter cartridge product (i.e., so it does not perform a filtration function).
Filter cartridges of predetermined length may be cut from an upstream filter cartridge perform of indefinite length having a melt-blown annular layer of non-woven melt-blown filtration zone fibers surrounding a melt-blown fibrous core element. According to the present invention, filter cartridges of very precise length can be achieved by subjecting the preform to forced cooling air to thereby minimize (if not eliminate entirely) significant filter cartridge shrinkage over time. In addition, a cutter assembly is mounted laterally of the preform, but is capable of rate-synchronized longitudinal movement with the preform during the cutting operation. In this manner, very precise cuts may be made through the preform which do not necessarily require downstream trimming in order to achieve precise nominal filter cartridge lengths.
These and other aspects and advantages of the present invention will become more clear from the following detailed description of the preferred exemplary embodiments thereof.